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Ubiquitin is a small protein consisting of 76 amino acids that is important for the regulation of protein function in the cell. Ubiquitylation and deubiquitylation are enzymatically mediated processes by which ubiquitin is covalently bound or cleaved from a target protein. These processes have been implicated in the regulation of many cellular functions including cell cycle progression, apoptosis, modification of cell surface receptors, regulation of DNA transcription and DNA repair. Thus, the ubiquitin system has been implicated in the pathogenesis of numerous disease states including inflammation, viral infection, metabolic dysfunction, CNS disorders, and oncogenesis (Clague et al., Physiol Rev 93:1289-1315, 2013). A number of ubiquitin-like (Ubls) molecules have been identified that regulate protein functions in cells in a similar manner to ubiquitin.
Ubiquitin and Ubls molecules are cleaved from proteins by enzymes called isopeptidases or deubiquitinating enzymes (DUBs), of which there are approximately 95 DUBs in human cells, divided into sub-families based on sequence homology: ubiquitin C-terminal hydrolases (UCHs), ubiquitin-specific proteases (USPs), ovarian tumour proteases (OTUs), Machado-Josephin domain proteases (MJDs), JAB1/MPN/MOV34 metalloproteases (JAMMs) or Sentrin-specific proteases (SENPs). DUBs can process ubiquitin or ubiquitin-like adducts. A number of DUBs have been linked to various diseases including cancer, inflammation, neurodegenerative diseases and anti-infectives (Kim K H et al., Curr Pharm Des. 2013; 19(22):4039-52; Nicholson B et al., J Biomol Screen. 2014 Mar. 14:19(7); 989-999; Ristic G et al., Front Mol Neurosci. 2014 Aug. 19; 7:72; Ashida H et al., Nat Rev Microbiol. 2014 June; 12(6):399-413. The ubiquitin C-terminal hydrolase (UCH) family, consisting of UCHL1, UCHL3, UCHL5, and BAP1, are cysteine proteases that operate through an active site thiol. UCHs are thought to be involved in the processing and recycling of ubiquitin and to preferentially cleave small protein substrates. UCHL1 is a 223 amino acid protein whose expression is normally limited to the brain, peripheral nervous system, ovaries and testis in mammals. However, UCHL1 has been reported to be up-regulated in several pathological conditions, including tumour tissues, COPD, stroke, Parkinson's disease, Alzheimer's disease, neuropathic pain or lysosomal storage disorders. Furthermore, UCHL1 functions as an oncogene in the progression of many cancers including breast, lymphoma, colorectal cancer, osteosarcoma, pancreatic and non small cell lung carcinoma and is inversely correlated with patient survival (Hurst-Kennedy et al., Biochem Res Int, 2012; Hussain et al., Leukemia 24:1641-1655, 2010). Pharmacological inhibition of UCHL1 would thus serve as novel treatment for pathologies such as cancers.
The ubiquitin-proteasome system has gained interest as a target for the treatment of cancer following the approval of the proteasome inhibitor bortezomib (Velcade®) for the treatment of multiple myeloma. Extended treatment with bortezomib is limited by its associated toxicity and drug resistance. However, therapeutic strategies that target specific aspects of the ubiquitin-proteasome pathway upstream of the proteaseome, such as DUBs, are predicted to be better tolerated (Bedford et al., Nature Rev 10:29-46, 2011). To date, although there have been a handful of inhibitors published in the literature (for a review see: Lill and Wertz, Trends in Pharmaceutical Sciences, 35 (4), 2014), there have been no reports of DUB inhibitors that have successfully entered the clinic. Thus, there is a need for compounds and pharmaceutical compositions to inhibit DUBs such as UCHL1, USP6 or USP30 for the treatment of cancer or other indications where DUB activity is observed. Alternative DUBs that have also been suggested to be potential therapeutic targets for the treatment of cancer include USP1, USP2, USP4, USP6, USP7, USP8, USP9x, USP10, USP11, USP13, USP14, USP17, USP28. Additional indications where DUB inhibition may prove beneficial include CNS disorders (e.g. USP30, USP14) or inflammation (e.g. A20, CYLD).